An absorbent article such as a disposable diaper generally includes: a front waistline portion to be fitted to the front waist of a wearer; a back waistline portion to be fitted to the back waist of the wearer; a crotch portion to be fitted to the crotch of the wearer; and leg-surrounding opening portions which open at both sides of the crotch portion. The front waistline portion and the back waistline portion are joined to each other at waist lateral-side portions.
In the steps of manufacturing a disposable diaper, a web is folded into two at the crotch portion by overlapping its first web to form the front waistline portion and its second web to form the back waistline portion with each other, and the web thus folded is joined at the left and right waist lateral-side portions, thereby forming the web into pants-type diapers. Ultrasonic joining using an ultrasonic joining apparatus is employed for the joining of the web at the left and right waist lateral-side portions (for example, Patent Document 1).
The ultrasonic joining apparatus includes: a rotating drum having an anvil formed thereon, the anvil being configured to form a joint pattern by which the front waistline portion and the back waistline portion are joined at the left and right waist lateral-side portions; and an ultrasonic horn configured to generate ultrasonic waves. The ultrasonic joining apparatus joins the first and second webs of the web, which is folded into two at the crotch portion, by pressing the ultrasonic horn intermittently against certain positions of the left and right waist lateral-side portions of the first and second webs while conveying the first and second webs between the rotating drum and the ultrasonic horn in a direction in which manufacturing processes flow.
The anvil includes: a projection being provided on the outer peripheral surface of the rotating drum and protruding in a normal direction of the rotating drum; and multiple protrusions protruding from the projection in the normal direction. When the ultrasonic horn is pressed against the anvil with the web interposed therebetween, the web is melted by being heated from the inside thereof, and is pressed against the protrusions protruding from the projection. As a result, a convexo-concave joint pattern is formed on the web.
Demand for diapers for adults has been growing in recent years. A diaper for adults is larger in size, and thus its manufacturing line is larger than a diaper for children. For example, since a waist lateral-side portion of the diaper for adults is longer than that of the diaper for children, the size of the anvil needs to be accordingly increased in length in a cross direction in a manufacturing line.
Thus, the following problems occur in the conventional ultrasonic joining apparatus. The ultrasonic horn is pressed against the anvil with the web interposed therebetween. Hence, if the anvil is large in size, damages caused by the mutual interference of vibrations generated by the ultrasonic horn are likely to be locally accumulated in the anvil or accumulated in a peripheral member connected to the anvil. This shortens the time to replace the anvil or the peripheral member, and increases manufacturing costs. Moreover, the damages on the anvil result in a reduction in yield rate.